"Its orbit brings it close to Earth, but also takes it way out past Jupiter. Such a vast orbit is similar to a comet's, not an asteroid's, which tend to be more circular, so people thought it was one that had shed all its ice deposits."

Don Quixote has a diameter of 19 kilometres and is the third largest known near Earth object (NEO) - bodies with orbits bringing them close to Earth.

Most NEOs are asteroids, but about five per cent are thought to be dead comets having run out of volatile ices, such as water and carbon dioxide, which boil off as they get close to the Sun, giving comets their spectacular coma and tail.

Images of Don Quixote taken by NASA's Spitzer Space Telescope, when its orbit brought it closest to the Sun, show it has a coma and faint tail.

"The power of the Spitzer telescope allowed us to spot the coma and tail, which was not possible using optical telescopes on the ground," says Emery.

"We now think this body contains a lot of ice, including carbon dioxide and/or carbon monoxide ice, rather than just being rocky."

Rock comet

Meanwhile Professor David Jewitt of University College London, told the conference, the 5-kilometre wide asteroid Phaethon has a comet-like tail.

However, unlike a comet Phaethon's tail isn't caused by the vaporisation of an icy nucleus, but by surface rocks becoming so hot during its close approach to the Sun, that they crack and crumble to dust.

The Earth ploughs through this dust trail every December, resulting in the Geminids meteor shower.

Astronomers have known for more than 30 years that Phaethon causes the Geminids, but Jewitt and colleagues using NASA's STEREO Sun-observing spacecraft, were the first to see it producing a comet-like tail.

According to Jewitt, Phaethon's Sun-grazing orbit causes it to reach temperatures of more than 700°C, launching small dust particles from its surface, which are picked up by sunlight and carried into the tail.

"By the shape of its orbit, Phaethon is definitely an asteroid," says Jewitt.

"But by ejecting dust it behaves like a rock comet."

Looks matter

Historically the definition of a comet and asteroid was based solely on appearance, according to Australian asteroid and comet hunter Dr Rob McNaught.

"If the object appears as a single light point, like a star when seen through the telescope, it was called a minor planet or asteroid," says McNaught.

"However, if it showed a diffuse appearance, a fuzzy coma surrounding the object, or a tail, it was by definition a comet."

According to McNaught, astronomers now have more accurate definitions, describing asteroids as a rocky bodies, while comets contain a lot of fine dust and volatile ices, which are sublimated by the heat of the Sun.

McNaught says scientists suspected Phaethon was different right from the outset.

"Phaethon's unusual because the Geminids meteors are one of the densest of meteoroids," he says. "They seem to be fairly hard silicate grains that penetrate quite deeply into Earth's atmosphere, more deeply than most meteors associated with comets."

"Referring to it as a rock comet seems a reasonable way to distinguish what's happening from the icy comets."

Evidence of hot springs discovered on the Saturnian moon Enceladus. Also; the Milky Way at least 50 per cent bigger than previously thought, and a new population of dwarf satellite galaxies discovered orbiting the Milky Way.